Canadian Patent CA 2294410 (Lauder) and Canadian Patent Application CA 2493340 (Day) describe operational difficulties experienced by state of the art reciprocating devices used to power chemical injection pumps on oil and gas wells. At low operating speeds and low operating pressures, the reciprocating devices can stall and become stuck as the operating valve passes through the middle position. Lauder describes a solution for the reciprocating devices that utilizes fluid pressure to move the device in a first direction and a spring to move in it in a second direction. Day describes a similar device adapted to move the device in both the first and second directions using fluid power.
In the prior art, the fluid used to actuate reciprocating movement is vented to the atmosphere through an exhaust. Typically, the supply fluid pressure must be a minimum of about 8-10 p.s.i. to overcome the biasing spring and operate such injectors.
In some existing systems, end of stroke detectors such as a limit switch shifts a supply valve to alternately pressurize different chambers which causes the reciprocating motion. This method typically requires a minimum supply pressure of about 20-30 p.s.i. to shift the supply valve. This method also requires venting to the atmosphere. Where the supply fluid is compressed air, this is not an issue, however, if the supply fluid is casing or well gas, venting to the atmosphere may be uneconomical and environmentally detrimental.
Therefore, there is a need in the art for a method of reciprocation which allows low-pressure operation, and which may limit or reduce atmospheric emissions of the supply fluid.